| 1. |
- Bosi, Ferdinando, et al.
(författare)
-
Late magmatic controls on the origin of schorlitic and foititic tourmalines from late-Variscan peraluminous granites of the Arbus pluton (SW Sardinia, Italy) : Crystal-chemical study and petrological constraints
- 2018
-
Ingår i: Lithos. - : Elsevier BV. - 0024-4937 .- 1872-6143. ; 308-309, s. 395-411
-
Tidskriftsartikel (refereegranskat)abstract
- Tourmalines from the late-Variscan Arbus pluton (SW Sardinia) and its metamorphic aureole were structurally and chemically characterized by single-crystal X-ray diffraction, electron and nuclear microprobe analysis, Mössbauer, infrared and optical absorption spectroscopy, to elucidate their origin and relationships with the magmatic evolution during the pluton cooling stages. The Arbus pluton represents a peculiar shallow magmatic system, characterized by sekaninaite (Fe-cordierite)-bearing peraluminous granitoids, linked via AFC processes to gabbroic mantle-derived magmas. The Fe2+-Al-dominant tourmalines occur in: a) pegmatitic layers and pods, as prismatic crystals; b) greisenized rocks and spotted granophyric dikes, as clots or nests of fine-grained crystals in small miaroles locally forming orbicules; c) pegmatitic veins and pods close to the contacts within the metamorphic aureole. Structural formulae indicate that tourmaline in pegmatitic layers is schorl, whereas in greisenized rocks it ranges from schorl to fluor-schorl. Tourmalines in thermometamorphosed contact aureole are schorl, foitite and Mg-rich oxy-schorl. The main substitution is Na + Fe2+ ↔ □ + Al, which relates schorl to foitite. The homovalent substitution (OH) ↔ F at the O1 crystallographic site relates schorl to fluor-schorl, while the heterovalent substitution Fe2+ + (OH, F) ↔ Al + O relates schorl/fluor-schorl to oxy-schorl. Tourmaline crystallization in the Arbus pluton was promoted by volatile (B, F and H2O) enrichment, low oxygen fugacity and Fe2+ activity. The mineralogical evolutive trend is driven by decreasing temperature, as follows: sekaninaite + quartz → schorl + quartz → fluor-schorl + quartz → foitite + quartz. The schorl → foitite evolution represents a distinct trend towards (Al + □) increase and unit-cell volume decrease. These trends are typical of granitic magmas and consistent with Li-poor granitic melts, as supported by the absence of elbaite and other Li-minerals in the Arbus pluton. Tourmaline-bearing rocks reflect the petrogenetic significance of contribution from a metapelitic crustal component during the evolution of magmas in the middle-upper crust.
|
|
| 2. |
- Grew, Edward S., et al.
(författare)
-
Fluor-elbaite, lepidolite and Ta–Nb oxides from a pegmatite of the 3000Ma Sinceni Pluton, Swaziland: evidence for lithium–cesium–tantalum (LCT) pegmatites in the Mesoarchean
- 2018
-
Ingår i: European journal of mineralogy. - : Schweizerbart. - 0935-1221 .- 1617-4011. ; 30:2, s. 205-218
-
Tidskriftsartikel (refereegranskat)abstract
- Mineral evolution is concerned with the timing of mineral occurrences, such as the earliest reported occurrences in the geologic record. Minerals containing essential Li have not been reported from rocks older than ca. 3000 Ma, thus the lithian tourmaline (fluor-elbaite) and mica (lepidolite) assemblage from a pegmatite near Zishineni associated with the ca. 3000Ma Sinceni Pluton presents unusual interest. Fluor-elbaite (0.75–0.98 F per formula unit) forms green crystals up to 50mm long. Spindle stage measurements give ω = 1.652(1), ε = 1.627(1) (589.3 nm). Optical absorption spectroscopy shows Fe and Mn are divalent; infra-red spectroscopy demonstrates the presence of Li and indicates the presence of (OH) at both the (OH) sites. Electron microprobe analysis of 330 points on several prisms, the largest of which is zoned in Fe and Ca, gives the following average and standard deviations in wt%: SiO2 37.29 (0.26), TiO2 0.05 (0.05), Al2O3 38.14 (0.35), Cr2O3 0 (0.02), MgO 0.02 (0.01), MnO 3.57 (0.25), FeO 2.48 (0.60), Na2O 2.48 (0.09), K2O 0.03 (0.12), CaO 0.77 (0.21), F 1.80 (0.11), Cl 0 (0.01) wt%. Nuclear reaction analyses gave Li2O 0.91 (0.04) and B2O3 10.55 (0.45). The empirical formula of fluor-elbaite was determined by integrating crystal-chemical data from electron microprobe analysis, nuclear reaction analysis, crystal structure refinement using X-ray diffraction, infra-red and optical absorption spectroscopy:X(□0.09Na0.77K0.01Ca0.13)Σ1.00 Y(□0.35Li0.59Mn2+0.49Fe2+0.33Al1.23Ti0.01)Σ3.00Al6(Si6O18)(BO3)3O3(OH)3O1[F0.92(OH)0.08]Σ1.00. The crystal structure of fluor-elbaite was refined to statistical indices R1 for 1454 reflections ∼2% using MoKa X-ray intensity data. Structural data confirm the presence of significant vacancies at the Y site. Micas include lepidolite in flakes several millimeters across that are veined and overgrown by fine-grainedmuscovite. Silica and (FeO+MnO) increase, and Al decreases with F, all giving tight linear fits for both micas taken together, suggesting bothmicas can be regarded as interstratified muscovite and lithium mica consisting of 35.2 wt% masutomilite containing nearly equal amounts of Mn and Fe, 52.8 wt% polylithionite and 11.9 wt% trilithionite. Muscovite and lepidolite contain <0.2 wt% and 0.7–2.25 wt% Cs2O and 1.0–1.1 wt% and 1.4–1.5wt% Rb2O, respectively. Other minerals include spessartine (e.g., Sps93Alm4Grs3) in scattered grains up to 0.5mm across and monazite.Oxides occur sparsely in muscovite, rarely in lepidolite, as grains up to 11 mm long, including fluorcalciomicrolite, columbite-(Mn) withNb>Ta, hübnerite(?) and a possible Pb-bearing microlite (Ta>Nb). The oxides, together with the muscovite, are interpreted to be related to later hydrothermal reworking of the primary lepidolite–fluorelbaite assemblage. Given the 2990 ± 43MaRb–Sr isochron and 3074 ± 4Ma evaporation Pb–Pb ages reported for the Sinceni Pluton and Rb/Sr mineral ages ranging from 2906 ± 31Ma to 3072 ± 33Ma reported for the pegmatites, the fluor-elbaite–cesian lepidolite–fluorcalciomicrolite-bearing pegmatite is the first reported occurrence of a lithian tourmaline and lepidolite in the geologic record, as well as one of the two earliest known examples of the lithium–cesium–tantalum (LCT) family of pegmatites. The Sinceni magma is most plausibly derived from a metasedimentary source by intrusion of hot mantle melts into the crust from below, thereby indicating that a “mature” continental crust existed in the Kaapvaal craton at ca. 3000 Ma.
|
|
| 3. |
- Nilsson, Charlotta, et al.
(författare)
-
A nuclear geochemical analysis system for boron quantification using a focused ion beam
- 2017
-
Ingår i: Journal of Radioanalytical and Nuclear Chemistry. - : Springer Science and Business Media LLC. - 0236-5731 .- 1588-2780. ; 311:1, s. 355-364
-
Tidskriftsartikel (refereegranskat)abstract
- Ion beam analysis has for decades been used as a tool for geochemical analysis of trace elements using both X-rays (particle induced X-ray emission) and nuclear reaction analysis. With the geoanalytical setup at the Lund Ion Beam Analysis Facility, the boron content in geological samples with a spatial resolution of 1 µm is determined through nuclear reaction analysis. In the newly upgraded setup, a single detector has been replaced by a double sided silicon strip detector with 2048 segments. After optimization, boron content in geological samples as low as 1 µg g−1 can be measured.
|
|
| 4. |
- Kristiansson, Per, et al.
(författare)
-
The implementation of a DSSSD in the upgraded boron analysis at LIBAF for applications in geochemistry
- 2014
-
Ingår i: Nuclear Instruments & Methods in Physics Research. Section B: Beam Interactions with Materials and Atoms. - : Elsevier BV. - 0168-583X .- 1872-9584. ; 332, s. 207-211
-
Tidskriftsartikel (refereegranskat)abstract
- Interest in high spatial resolution boron analyses from a geochemical perspective is related to the recognition of boron as an important tracer of chemical recycling in the Earth, due to the high solubility of boron in aqueous fluids and silicate melts. Although boron is not a nominal component in common silicates they may still contain significant B-concentrations and hence constitute important boron reservoirs in the deeper parts of the Earth. Boron analyses have been performed at the Lund Ion Beam Analytical Facility for almost 20 years. For the analysis the nuclear reaction p+B-11 is used with beam energy just below 700 keV where the reaction has a broad resonance. In this paper we describe an upgrade of the system with a double sided silicon strip detector, which allows for much higher count rates compared to the old annular surface detector based system. A gain close to 20 in the data rate allows for high resolution mapping of boron distributions in crystals. This is illustrated by a number of examples. In addition, the detection limits for boron in geological samples are improved, now around 5 ppmw. In this work we address issues with data quality, especially charge normalization, lifetime correction and subtraction of different background components. (C) 2014 The Authors. Published by Elsevier B.V.
|
|
| 5. |
- Hålenius, Ulf, et al.
(författare)
-
Coordination of Boron in Nominally Boron-Free Rock Forming Silicates: Evidence for Incorporation of BO3 groups in Clinopyroxene
- 2010
-
Ingår i: Geochimica et Cosmochimica Acta. - : Elsevier BV. - 0016-7037 .- 1872-9533. ; 74, s. 5672-5679
-
Tidskriftsartikel (refereegranskat)abstract
- To explore mechanisms of B-incorporation in common chain silicates we have investigated synthetic diopside samples produced under boron-saturated conditions by 11B and 29Si magic-angle spinning (MAS) NMR and single-crystal NRA, FTIR, EMP and XRD/SREF techniques. Our samples contain 0.14–0.65 wt.% B2O3. NMR reveals that B is predominantly present in trigonal coordination in the clinopyroxene structure. This observation is supported by vibrational bands characteristic for B–O stretching in BO3 groups in the range 1250–1400 cm−1 in polarised single crystal FTIR-spectra. Single crystal structure refinements suggest that boron replaces Si at the T site. Combined, these results suggest that boron replacement for Si at the T-site leads to disruption of one of the T–O bonds of the nominal clinopyroxene structure resulting in replacement of SiO4 tetrahedra by BO3 groups. Our results show that high concentrations of boron can be incorporated in the nominally boron-free diopside. Elevated B-concentrations in the present calcic clinopyroxenes are accompanied by modifications of the diopside crystal structure involving the breaking of one T–O bond and simultaneous formation of vacancies at the octahedral M2 site. These structural modifications destabilize the structure and constitute thereby limiting factors for incorporating higher boron concentrations in diopside.
|
|
